A disposable body warmer is a body warmer utilizing heat generation due to the oxidation action of iron powder, and is usually formed by accommodating, and packaging airtightly an inner bag that accommodates, in an air-permeable bag of a non-woven fabric, paper or the like, an exothermic composition containing iron powder as an exothermic element, a salt as an oxidation catalyst, activated carbon for intake of oxygen, water for oxidizing iron, a water-retaining agent for retaining water, and the like, in an outer bag made of an air-impermeable film for cutting off contact with air in an unused state.
A disposable body wanner outer bag is required to be excellent in gas barrier property that inhibits permeation of air, especially, oxygen gas, water vapor, and the like. If the disposable body warmer outer bag is poor in gas barrier property against oxygen gas and water vapor, any gas and water vapor in the disposable body wanner outer bag escape outside during storage for a long period to thereby produce a depressurized (vacuum) state, and therefore the outer bag is depressed and thus is not preferable in terms of appearance. In addition, when such a disposable body warmer outer bag in a depressurized (vacuum) state is stored for a long period and thereafter used for a disposable body wanner, the duration of heat generation is often short. As the air-impermeable film forming the disposable body warmer outer bag, a multilayer film is generally used where a gas barrier layer is provided on a sealant layer and a heat-resistant resin layer is provided on an outermost layer. Two of such air-impermeable multilayer films are superposed and the peripheries of the sealant layers located inward are mutually heat-sealed to form a bag, thereby producing the disposable body warmer outer bag.
As the air-impermeable film forming such a disposable body warmer outer bag, for example, an air-impermeable multilayer film has been used where a coating film of polyvinylidene chloride is provided on one surface of a substrate film such as a polyolefin-based resin film and a heat-sealing resin layer of an ethylene-vinyl acetate copolymer or the like is further provided thereon. The air-impermeable multilayer film, however, has the problem of deterioration in heat generation-maintaining property due to storage for a long period. In order to solve the problem, an air-impermeable multilayer film is proposed where a polyethylene terephthalate film covered with an aluminum oxide layer, a polyethylene film as an adhesion layer, and an ethylene-vinyl acetate copolymer as a heat-sealing resin layer are provided (PTL 1).
Meanwhile, it is known that, when a disposable body warmer is stored in an unused state for a long period, a trace amount of hydrogen gas is generated. A disposable body warmer outer bag is swollen by hydrogen gas, and is not preferable in terms of appearance. In addition, if marginally swollen, the outer bag is broken, and an exothermic composition accommodated in an inner bag is oxidized to generate heat, thereby causing the function as a disposable body warmer to be impaired. Therefore, the disposable body warmer outer bag is required to have a hydrogen permeation property so that the outer bag is not swollen during storage for a long period. The disposable body warmer outer bag, however, is also required to be excellent in gas barrier property that inhibits permeation of oxygen gas, water vapor and the like, as described above. Therefore, if the gas barrier property of the air-impermeable film forming the disposable body warmer outer bag is enhanced, a hydrogen gas permeation property is deteriorated to result in swelling during storage for a long period. The Applicant has then proposed, as one that satisfies both conflicting conditions of a gas barrier property against oxygen gas and water vapor and a hydrogen gas permeation property, a multilayer film including a vapor-deposited layer obtained by vapor-depositing a metal or metal oxide on a substrate, and a polyvinylidene chloride layer obtained by coating the substrate with polyvinylidene chloride, and a disposable body warmer where the multilayer film is used for an outer bag has been marketed by the Applicant.
The multilayer film including the vapor-deposited layer and the polyvinylidene chloride layer not only is excellent in gas barrier property that inhibits permeation of oxygen gas, water vapor and the like, but also can allow swelling due to hydrogen gas generated during storage to be prevented. Thus, the multilayer film including the vapor-deposited layer and the polyvinylidene chloride layer has no large problem when used as an outer bag for a disposable body warmer, but the Applicant has made studies and as a result has found the following points to be improved.
(1) Durability during Distribution as Product
An outer bag may be subjected to external high impact, and therefore an outer bag made of an air-impermeable film is demanded to have seal strength between respective layers and impact resistance so that the outer bag is neither opened nor ruptured even when subjected to impact.
(2) Weather Resistance during Display at Store Front
An outer bag may be exposed to ultraviolet light such as sunlight, and therefore an outer bag made of an air-impermeable film is demanded to have weather resistance so that the change in appearance such as discoloration is not caused even after exposure to ultraviolet light such as sunlight.
While the multilayer film including the vapor-deposited layer and the polyvinylidene chloride layer not only is excellent in gas barrier property, but also can allow swelling due to hydrogen gas to be prevented, as described above, it has been found to have room for improvement in seal strength, impact resistance and weather resistance.
As described above, various multilayer films for disposable body warmer outer bag are proposed, but a multilayer film for a disposable body warmer outer bag, and a disposable body warmer are required which can ensure an optimum gas permeation property and gas barrier property as a product over a long period and which are excellent in seal strength, impact resistance and weather resistance.